This invention relates to fused silica glass and, in particular, to burners for producing silica soot from which such glass can be made. As used herein, the term xe2x80x9csilica glassxe2x80x9d includes glass which is pure or may contain one or more dopants, as does the term xe2x80x9csilica sootxe2x80x9d.
An effective method for making fused silica glass comprises the steps of: (1) generating silica soot particles using soot producing burners, and (2) collecting and consolidating the particles on a rotating substrate to form a glass xe2x80x9cboulexe2x80x9d. Such boules can have diameters on the order of five feet (1.5 meters) and thicknesses on the order of 5-10 inches (13-25 cm). The process is typically carried out in a furnace which has a rotatable base, an outer ring wall, and a crown which carries the soot producing burners.
FIG. 1 shows the front face 8 of a soot producing burner 7 which has been used in the past to produce fused silica boules. This burner has five zones or regions 10, 12, 14, 16, and 18 through which gases of different compositions pass to (1) supply the raw material(s) from which the soot particles are produced, and (2) generate a flame of suitable size and temperature to (a) convert the raw material(s) into soot particles and (b) generate sufficient heat to consolidate the particles as they are collected at the surface of the boule.
For the burner of FIG. 1, region 10 is referred to as the xe2x80x9cfume tubexe2x80x9d and carries, for example, a mixture of nitrogen gas and a vaporized silicon-containing compound, regions 12 and 18 are known as the inner and outer shields, respectively, and carry oxygen, and regions 14 and 16 are referred to as the xe2x80x9cpremix ringsxe2x80x9d and carry a mixture of fuel (e.g., methane) and oxygen. The diameter of outer shield 18 is typically about 1.1 inches (2.8 cm), while the overall dimensions of front face 8 are typically about 3.4 inches by 3.4 inches (8.5 cm by 8.5 cm).
Historically, the vaporized silicon-containing compound supplied to fume tube 10 was silicon tetrachloride or a mixture of silicon tetrachloride and chlorides of other materials, e.g., titanium tetrachloride, when a doped glass was desired. As a result of environmental concerns, silicon tetrachloride has now been replaced with halide-free, silicon-containing compounds, of which octamethylcyclotetrasiloxane (OMCTS) is a particularly preferred example since in addition to providing silicon, it is also provides energy for the burner""s flame. In the same manner, organometallic compounds have been substituted for chloride compounds in the production of doped glasses.
FIG. 2 shows the manner in which burners of the type shown in FIG. 1 have been positioned relative to the furnace""s crown 20. With regard to the present invention, it is significant to note that burner 7 is spaced from the outer face 22 of the crown (the xe2x80x9ccoldxe2x80x9d face of the crown) by gap 24. This gap, which in practice is about a quarter inch in height, allows air to be inspirated into the furnace so as to cool burner hole 26 and prevent soot buildup on the walls of the hole. The entrained air also ensures that complete combustion of the fuel occurs in burner flame 38.
In addition to illustrating the spatial relationship between burner 7 and crown 2, FIG. 2 also shows the connection of feed lines 28, 30, and 32 to the burner, as well as lines 34 and 36 which carry cooling water to and from the burner.
Although burners and burner/crown configurations of the type shown in FIGS. 1 and 2 have worked successfully in practice, they have had some drawbacks. In particular these burners have suffered from the following problems:
(1) Maintenance Problem
Because of their relatively large frontal areas exposed to furnace conditions, the previously used burners tend to collect deposits on burner face 8 which must be removed to avoid variations in the burner""s flame characteristics and/or the soot produced by the burner. In particular, large frontal areas make a burner subject to recirculation effects whereby soot which is not deposited on the boule recirculates back and fouls the face of the burner.
(2) Water Cooling Problem
The large frontal areas of the previously used burners also result in substantial heat transfer from the hot furnace to the burner, thus requiring water cooling of the burners. This is especially so in view of the fact that the burners are made out of aluminum. (It should be noted that the heat transfer occurs both through burner hole 26 and through the crown material itself since the crown is desirably made as thin as possible.) The need for water cooling makes the burners more complex to build and operate.
(3) Furnace Atmosphere Control Problem
The inspiration of air through the burner holes in the crown makes it more difficult to control the composition of the atmosphere within the furnace. Variations in the furnace atmosphere can result in variations in the properties (e.g., hydrogen content) of the glass boules produced by a furnace, both between different parts of a single boule and between different boules.
(4) Emissions Problem
The inspiration of air through the burner holes can also result in elevated levels of NOx in the exhaust gases exiting the furnace since N2 is the major constituent of the inspirated air and furnace temperatures are high enough for NOx production, e.g., above 1600xc2x0 C.
(5) Energy Consumption Problem
Inspiration of ambient air through the burner holes leads to an increase in the amount of energy which must be inputted to the furnace to keep it at its operating temperature.
(6) Potential Safety Problem
The feeding of a premix of fuel and oxygen to regions 14 and 16 makes these regions and the feed lines leading thereto susceptible to flame flashback.
As discussed below, the burners of the present invention address and provide solutions to each of these problems.
The use of halide-free, silicon-containing compounds to form fused silica glasses by soot deposition is discussed in Dobbins et al., U.S. Pat. No. 5,043,002, and Blackwell et al., U.S. Pat. No. 5,152,819. The incorporation of a dopant, specifically, titanium, in such glasses is discussed in Blackwell et al., U.S. Pat. No. 5,154,744. The contents of these prior patents are incorporated herein by reference.
PCT Patent Publication No. WO 97/22553, published on Jun. 26, 1997, discloses soot producing burners which can be used with halide-free, silicon-containing compounds such as octamethylcyclotetrasiloxane (OMCTS). The halide-free, silicon-containing compound is preferably provided to the burner as a liquid, atomized in the burner by an integral atomizer, and then directly converted into soot particles by the burner""s flame. See also pending U.S. applications Ser. No. 08/767,653 and Ser. No. 08/903,501, filed Dec. 17, 1996 and Jul. 30, 1997, respectively, the contents of both of which are incorporated herein by reference.
Miller et al., U.S. Pat. No. 5,110,335 discloses a burner for producing soot from silicon tetrachloride which includes an ultrasonic nozzle which when operated at a frequency of 120 kilohertz converts liquid silicon tetrachloride into a fine mist.
Brown et al., U.S. Pat. No. 5,092,760 discloses an oxygen/fuel burner which atomizes liquid fuel by means of an integral atomizer. Brown et al., U.S. Pat. Nos. 5,405,082 and 5,560,758, disclose oxygen/fuel burners for use in glass conditioning. These burners employ a tube-in-tube construction and, during use, are sealed to the wall of a glass distribution channel. Brown et al., U.S. Pat. No. 4,986,748 discloses a further construction for an oxygen/fuel burner. Significantly, with regard to the present invention, the burners of these various Brown et al. patents are concerned with heat production, not with the production of silica soot. Among other things, such heat producing burners do not have to be concerned with soot build-up on the burner face or with the adverse effects of the burner""s internal operating temperature on the heat-sensitive raw material(s) used to produce silica soot.
In view of the foregoing, it is an object of the present invention to provide improved burners for producing silica soot. More particularly, it is an object of the invention to provide improved burners which overcome some and preferably all of the above problems of previously used soot producing burners.
The invention achieves these and other objects by providing soot producing burners and furnaces employing such burners which have some or all of the following properties:
(1) The burner uses a tube-in-tube design so as to reduce the frontal area of the burner and thus minimize the soot build-up problem. For example, the frontal area of a burner constructed in accordance with the invention can be about 0.32 square inches (2.1 square centimeters) whereas burners of the type shown in FIGS. 1 and 2 had frontal areas of about 1.8 square inches (11.4 square centimeters).
The tube-in-tube design produces a plurality of passages for carrying liquid and/or gaseous materials, namely, a first passage constituting the bore of the innermost (first) tube, a second passage defined by the outer surface of the first tube and the inner surface of the next innermost (second) tube (the first pair of tubes), a third passage defined by the outer surface of the second tube and the inner surface of the third tube (the second pair of tubes), and so on. In this way, xe2x80x9cnxe2x80x9d tubes define xe2x80x9cnxe2x80x9d passages.
Not all passages need extend throughout the entire length of the burner. For example, as discussed below, the first tube may end prior to the face of the burner, whereupon the contents of the first passage merge with the contents of the second passage. The innermost passage at the face of the burner is then defined by the inner surface of the second tube, rather than the inner surface of the first tube.
(2) To provide a focused, relatively uniform flow pattern, one or more of the passages produced by the tube-in-tube design can include flats in, for example, the vicinity of the burner""s face which serve to guide the flow of gas out of the burner. These flats can be oriented at an angle with respect to the burner""s face, e.g., at an angle of approximately 75 degrees with respect to the burner""s axis (see FIG. 4). The flats are preferably formed on the outer surface of the inner tube of the pair of tubes which defines the passage. Alternatively, although less preferred for manufacturing reasons, the flats can be formed on the inner surface of the outer tube of the pair of tubes which defines the passage. It should be noted that in either case, sizing the tubes so that they make contact at the corners of the flats in the case of flats on the inner tube or at the centers of flats in the case of flats on the outer tube results in a passage of limited cross-sectional area. This contacting also aids in centering the inner tube within the bore of the outer tube.
(3) Flats can also serve to atomize a liquid raw material, e.g., liquid OMCTS or a mixture of liquid OMCTS and one or more liquid dopants. In particular, in accordance with these aspects of the invention, the liquid raw material is subjected to shear forces as it passes through a restriction zone formed by flats. Preferably, the passage which carries the liquid raw material has a cross-sectional area which decreases as the raw material approaches the restriction zone and a cross-sectional area which increases after the raw material has passed through the restriction zone. Such changes in cross-sectional areas can be achieved by, for example, tapering one or both of the tube surfaces which define the passage. In addition to the restriction zone, the passage carrying the liquid raw material preferably merges with a passage carrying gas, e.g., a passage carrying oxygen, downstream of the restriction zone to further enhance the atomization of the liquid raw material.
In comparison to orifices, flats have the advantage of being able to achieve atomization for low flow rates of a liquid raw material, e.g., flow rates less than about 10 grams/minute.
(4) To minimize soot deposition on the face of the burner, it has been found that the passage which provides soot producing raw material(s) to the burner flame needs to extend beyond the face of the burner. Preferably, this passage is the center passage of the burner and the passages surrounding the center passage, which carry fuel and oxygen, are angled towards the center passage to further reduce soot build up on the burner face.
(5) The burners are sealed to the crown of the furnace so as to substantially completely eliminate inspiration of air into the furnace at the locations of the burners. Preferably, inspiration is completely eliminated although in some cases, minor amounts of leakage of air at the crown/burner interface can be tolerated without encountering the various problems discussed above which result from large amounts of air passing through a burner hole.
(6) Cooling of such sealed burners is accomplished by the flow of gases through the burner. In particular, oxygen is flowed through the outermost passage of the burner where the greatest amount of heat transfer from the crown occurs. In addition, the burner can be equipped with an external air cooled jacket to further reduce its internal operating temperature.
(7) The burner has completely separate passages for fuel (e.g., methane, natural gas, hydrogen, etc.) and oxygen so that the mixing of fuel and oxygen does not occur until after these materials have exited the burner face, thus eliminating the possibility of flashback. That is, the burner of the invention uses xe2x80x9cnozzle mixingxe2x80x9d of the fuel and oxygen rather than xe2x80x9cpremixingxe2x80x9d of these materials.
By means of these features, the invention provides improved burners which are economical to build, use, and service, and which allow for more efficient and controlled production of fused silica boules.